In Drosophila virilis, about three-fourths of genes coding for 28S ribosomal RNA are interrupted by an intervening sequence. Many of these interruptions are 5 kb in length, and most of the rest are 10kb long; rDNA repeats containing these are evidently not transcribed. Sequences homologous with elements of rDNA intervening sequences are manifold elsewhere in the genome, and are present to various extents in rDNA intervening sequences of diverse other higher Diptera. There are three major aspects of the proposed work: 1) a determination of the structural and functional relationships of intervening sequences to the rest of rDNA to learn why most, but not all, 28S rRNA coding regions are discontinuous and not expressed; 2) an analysis of sites and extents of nucleotide sequence homology among rDNA intervening sequences in diverse Diptera to learn of the origins and patterns of evolution of rDNA intervening sequences; and 3) an examination of the structure of chromosomal distribution of non-rDNA elements of the Drosophila genome that exhibit sequence homology with the 28S gene interruptions, and of the structure and function of their flanking sequences, to determine the role of these elements in genome organization and/or expression.